Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers are used.
Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little wet strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water. U.S. Pat. No. 5,585,456. Wet strength of ordinary paper is only about 5% of its dry strength. To overcome this disadvantage, various methods of treating paper products have been employed.
Wet strength resins applied to paper are either of the “permanent” or “temporary” type, which are defined by how long the paper retains its wet strength after immersion in water. While wet strength retention is a desirable characteristic in packaging materials, it presents a disposal problem. Paper products having such characteristics are degradable only under undesirably severe conditions. While some resins are known which impart temporary wet strength and thus would be suitable for sanitary or disposable paper uses, they often suffer from one or more drawbacks. For example, their wet strength is generally of a low magnitude (about one-half of the level achievable for permanent-type resins); they are easily attacked by mold and slime; or they can only be prepared as dilute suspensions.
There is a continuing need in the art for methods of imparting appropriate levels of wet strength to paper products.